This invention relates to mechanism for forming a leno selvage in a loom and the method for forming such a selvage.
One of the more successfull leno devices currently being used includes a rotating disc having a pair of spaced guide holes. A pair of leno threads extend through the guide holes from a pair of supply packages mounted on the disc to the selvage. For each rotation of the disc, the two leno threads occupy upper and lower shed positions alternately. Since the guide eyes are mounted on the disc, the movement of the leno threads between upper and lower shed positions is harmonic. Although vertical movement of the leno threads reaches the zero point at the extreme upper and lower positions, it is not a true dwell. Weft insertion in the loom occurs from a period of time before the leno threads reach their extreme vertical or "open" shed positions and a period of time after. The threads will cross after the weft has been inserted and as they move towards opposite vertical positions, a new weft is inserted. The timing of the various loom functions such as weft insertion has to be done within the bounds of the natural harmonic motion of the leno threads. In the past, this natural harmonic motion has been more than adequate. The leno shed is opened to a greater extent than necessary so that the leno threads will be spaced a sufficient amount for a longer period of time. With the advent of higher speed looms, less time is available for opening the leno shed so that there is a greater need to keep the leno shed opening to a minimum. It is also desired that the leno threads cross quickly so that the filling ends will be locked in place before beat-up. There are other leno devices which control the leno shed opening with cams so that the leno threads will dwell in the open position and then cross relatively quickly. This solution is not as good for the loom speeds which are contemplated. The cam has to be larger, there are more moving parts, the moving parts have greater mass and due to the mass of the moving parts, there are limitations in designing a cam contour. Accelerations have to be gradual when there is a change in direction of cam followers and associated parts.